compositing-mode
:copy
means that the source is written to the destination ignoring the existing values. If the source has alpha and the target does not, that has the effect of converting semi-transparent source to solid.
:copy
is especially useful for creating transparent and semi-transparent pixmap ports, which can be displayed directly or converted to images by
make-image-from-port
.

Further
compositing-mode
values are supported on later versions of Cocoa and GTK+.